In modern high efficiency gas turbines, shrouded blades are employed which, during operation, are subjected to hot gases with temperatures of more than 1200° K and pressures of more than 6 bar.
A basic configuration of a shrouded blade is shown in FIG. 1. The blade 10 comprises a blade airfoil 11 which merges, in the downward direction, via a blade shank 25 into a blade root 12. At the upper end, at a blade tip or airfoil tip, the blade airfoil 11 merges into a shroud section 21 which, in the case of a complete blade row and together with the shroud sections of the other blades, forms a closed annular shroud. The blade airfoil has a spanwise direction extending from the blade shank to the blade tip. As, when the blade is inserted in a turbine, the spanwise direction is arranged in a radial direction of the turbine cross section, this direction may hereinafter also be referred to as a blade radial direction. The blade airfoil 11 has a leading edge 19, onto which the hot gas flows, and a trailing edge 20. Within the blade airfoil 11 are arranged a plurality of radial coolant ducts 13, 14 and 15 which are connected together, in terms of flow, by means of deflection regions 17, 18 and form a serpentine with a plurality of windings (see the flow arrows in the coolant ducts 13, 14, 15 of FIG. 1).
Because the coolant passes once through the serpentine-type sequentially connected coolant ducts 13, 14, 15, the coolant flows with increasing temperature through the coolant ducts and attains the maximum temperature in the last, trailing edge 20 coolant duct 15. The trailing edge 20 of the blade 10 can therefore, under certain operating conditions, attain excessively high coolant and blade material or metal temperatures. An incorrect matching of the metal temperature over the axial length of the blade can lead to high temperature creep and, in consequence, to deformation of the trailing edge 20. In the case of a shrouded blade, such as is shown in FIG. 1, tipping of the shroud segments 21 in the axial, radial and peripheral directions can occur as a secondary effect of the trailing edge deformation. The tipping of the shroud segments 21 can lead to opening of the gaps between individual shroud segments, which permits the entry of high temperature hot gas into the shroud space. As a consequence of this, the temperatures of the shroud metal can be significantly increased and rapidly introduce shroud creep and, finally, lead to high temperature failure of the shroud.
In the publication U.S. Pat. No. 4,278,400, cited at the beginning, a blade cooling supply has been proposed for blades with cooled tips and finely distributed cooling openings at the leading edge (film cooling). An ejector is arranged transverse to the flow direction of the main cooling flow at the end of a 90° deflection of the main cooling flow and, through this ejector, an additional flow of cooler coolant is injected into the coolant duct which runs along the trailing edge. The ejector can be supplied with coolant via a duct running radially through the root. The coolant emerging from the nozzle of the ejector with increased velocity can generate a depression, which can draw heated coolant from the coolant duct of the leading edge into the coolant duct of the trailing edge. Approximately 45% of the coolant flowing along the leading edge emerges through the cooling openings on the leading edge. 40% is induced by the injector. The rest emerges through cooling openings at the blade tip.
Due to the injector, the pressure relationships and flow relationships in the coolant duct can change relative to a configuration with simple supply through the inlet of the coolant duct on the leading edge. A balance between the coolant emerging at the leading edge for film cooling and the coolant induced by the injector will likely not exist, absent a completely new blade cooling design layout, which can be difficult to match to the changing requirements. The injector principle and the associated generation of depression are not suitable for blades without leading edge film cooling and blades with cooled shroud.